Lifting vessel and method for positioning, lifting and handling a platform deck and a jacket

ABSTRACT

A lifting vessel ( 1 ) consisting of a lower U-shaped pontoon foundation ( 2   a,    2   b,    2   c ), a number of vertical columns ( 5 ) attached to the pontoon foundation and extending upwards and through the water surface, and each column ( 5 ) being free-standing above the pontoon foundation ( 2   a,    2   b,    2   c ). Methods for positioning, lifting and handling of a platform dock and a platform jacket is also described.

The present invention is related to a lifting vessel having a U-shapedpontoon, with two longitudinal and one transversal pontoon, and columnsconnected to the pontoon extending upwards and through the watersurface.

The present lifting vessel is designed with a hull where the buoyancycan be adjusted by ballasting/deballasting, for lifting andtransportation operations at sea.

The invention includes also methods for positioning, lifting andhandling a platform deck and a jacket, with the help of the liftingvessel of the present invention.

In connection with offshore activities such as gas and oil exploitationit is usual to install platforms on the field. These platforms oftenconsist of large and heavy platform substructures fixed to the seabed.Such a platform substructure is normally a so-called “jacket”, which isa steel truss structure. On top of for example a jacket it is usual toplace a platform deck, which is used in connection with drilling andproduction. The deck also often includes living quarters.

To transport and install the jacket and the platform deck describedabove, for example barges have been used to transport the jacket andplatform deck out to the field, and large crane vessels have been usedto install the platform on the field.

Heavy lift vessels using ballast to vary their draft have also been usedto transport and install platforms offshore.

There are today a great number of offshore platforms installed toexploit oil and gas. When the oil and/or gas reservoirs are fullyexploited the life span of the platform is usually over and it would inmost cases be appropriate to remove the platform.

Some platforms are already removed, and removal of platforms willcontinue at an increasing pace the coming years.

The traditional way of removing platforms is to use large ocean goinglifting cranes. The platform needs to be very thoroughly prepared priorto removal, and it must be cut into smaller parts since even the largestlifting crane vessels have limited lifting capacity. The same goes forthe platform substructure (the jacket).

These operations are time consuming and costly, not only because thelifting cranes are large, expensive and need a large crew, but alsobecause cutting a platform to smaller pieces in open sea is a verycomplicated task. It is also a risky operation.

The new technology, as described in this application, can be describedas “single lift technology”, and will reduce the costs considerably. Itwill also make the operations less risky than present alternatives.Within the category “single lift technology” there are three otherconcepts that the applicant is aware of at the moment:

“Offshore Shuttle” is a vessel planned built as a frame work structure.The vessel has a significant length and the lifting of for example aplatform deck is based on crossbeams spanning across the structure.

“Master Marine” is developing a U-shaped semi submersible withdeck-structure connecting the top of columns. Lifting is based on loadtransfer to the deck-structure.

“Versatruss” is a concept involving two separate barges each supportingits own lifting frame. By pulling the barges together after positioningthe lifting frames beneath the lifting points on the platform deck, thelifting of the deck can be performed. This method has already been usedto remove small platform decks in calm waters.

One object of the present invention is to accomplish a removal operationof a platform in a fast and cost effective manner without cutting eitherthe deck or the jacket into smaller parts. The removal operation shallbe performed in a safe way where the safety of the operators isaccomplished in the best possible way.

Another object of the present invention is that the lifting vessel is asflexible as possible and that it can be easily adjusted to fit differentsized platform decks. Further the lifting vessel shall be able to liftand handle jackets of different sizes. In accordance with the inventionthe lifting vessel, a so-called Multi Purpose Unit, MPU, which also cantransport e.g. the platform deck to shore, and then transfer the deck toa barge or a pier suitable to the vessel.

Another object is that the lifting vessel shall be able to betransported on a heavy lift vessel, to reduce the travel time to distantdestinations like the Gulf of Mexico and the Persian Gulf.

Another object of the lifting vessel is that it shall also be able toinstall platforms, which basically is the reverse of removal. Thelifting vessel should furthermore be applicable to a range of purposeswhere a large lifting capacity is required.

The objects described above is achieved according to the invention bylifting vessel with a U-shaped pontoon, a number of columns connected tothe pontoon and extending upwards towards and through the water surface,characterised by the columns not being structurally connected above thepontoon.

Preferred embodiments of the lifting vessel is further described in theclaims 2 to 12.

The objects of the invention is further achieved by methods ofpositioning, lifting and handling of a platform deck and a jacketaccording to claims 13 and 14.

The present invention is described below with the help of examples ofuse and with references to the figures, where:

FIG. 1a shows a lifting vessel with attached lifting gear according tothe present invention,

FIG. 1b shows the lifting vessel according to the present invention,

FIG. 2 shows the lifting vessel positioned around a jacket with aplatform deck,

FIG. 3 shows a steel tubular rotation beam for lifting and rotating ajacket structure,

FIG. 4 shows a device for lifting and rotating a jacket structure forinstallation or removal,

FIGS. 5a- 5 c show the vessel in connection with lifting and rotating ajacket structure where a special “cradle” is used,

FIG. 6 shows lifting frames for lifting of preferably a platform deck,

FIG. 7 shows hydraulic jacks for operating the lifting frame, situatedbetween the lifting vessel and the inclined legs of the lifting frameand the figure also shows the steel tubular beam for lifting androtation/removal of a jacket structure,

FIG. 8 shows a hydraulic lock bolt system for locking of the liftingframe in a certain position to a guide rail connected to the liftingvessel,

FIG. 9 shows one first alternative for a connection between the liftingframe and the jacket structure for removal of a platform deck,

FIGS. 10a and 10 b show a second alternative for a connection betweenthe lifting frame and the jacket structure for removal of a platformdeck,

FIGS. 11a and 11 b show a third alternative for a connection between thelifting frame and the jacket structure for removal of a platform deck,

FIGS. 12, 13, 14 and 15 show step by step the operation sequence forremoval of a platform deck with the help of the lifting vessel of thepresent invention, and

FIGS. 16, 17, 18, 19 and 20 show step by step the operation sequence forremoval of a jacket structure with the help of the lifting vessel of thepresent invention.

The lifting vessel according to the present invention will now bedescribed with reference to the figures, especially FIGS. 1a and 2.

The lifting vessel 1 (MPU) is, according to the present invention,developed as a floating concrete hull with a U-shaped pontoon foundation2 containing two longitudinal pontoons 2 a, 2 b and a transverse pontoon2 c, and with columns 5 through the water surface for hydrostaticstability and optimal behaviour in the sea. The columns 5 are notconnected structurally at the top, which is made possible by a rigid androbust hull structure. A brim 3 along the lower edge of the pontoonfoundation improves further the behaviour of the vessel in the sea. Thevessel 1 is specially developed for operations offshore. The U-shape ofthe pontoon foundation 2 a, 2 b, 2 c enables the vessel to positionitself around a platform being installed or a platform being removed, beit the platform deck or a platform substructure. The lifting operationis performed according to Archimedes' principle byballasting/deballasting the vessel 1. The lifting is mainly performedvertically, but the vessel 1 can be inclined in all directions to enablespecial lifting operations.

Positioning of the vessel 1 is considered done by tugs, but thrusterscan be installed to make the vessel 1 self-propulsive. The vessel 1 isdesigned to operate in all oceans in all parts of the world. The vessel1 is also designed to be transported on a heavy lift ship to easetransportation over large distances.

The vessel 1 is equipped with devices specially fitted for theoperations the vessel 1 is intended for. Installation and removal ofplatforms (platform decks and platform substructures) for the oil andgas industry are examples of operations the vessel 1 is intended for.

Installation and removal of platform substructures are mentioned aboveas fields of operation for the lifting vessel 1 of the presentinvention. The vessel 1 will now be described in relation to theseoperations, especially in connection with the handling of jackets. Steeljackets are widely used all over the world in the oil and gas industryas substructure for offshore oil and gas production units. There arealso many other situations where a jacket structure is suitable as asupport structure. There will be a market for both installation andremoval of jackets in the future. Below is described operationsconcerning removal of a jacket. For installation the operations will beperformed in the reverse order.

Lifting brackets 25 are attached to the jacket legs on one side of thejacket at a certain, pre-established height. A circular tubular rotationbeam 22 is fixed to the top of the transverse pontoon 2 c of the liftingvessel 1. The lifting vessel 1 is positioned around the jacket with thehelp of tugs and active use of a lifting frame 12. This lifting framewill be described more thoroughly later in connection with liftingdevices for positioning and lifting of a platform deck. The vessel 1 ishauled to a position where the transverse pontoon 2 c of the vessel 1 ispositioned close to the side of the jacket where the lifting brackets 25are attached. The lifting vessel is ballasted to the desired draft andinclination of heel so that the tubular rotation beam 22 connects withthe lifting brackets 25, see FIG. 4, concurrent with the lower edge ofthe transverse pontoon 2 c bear against the jacket legs with fendersbetween them. The lifting brackets 25 are locked to the tubular rotationbeam 22 and by deballasting the lifting vessel 1 the jacket is lifted.When the jacket is lifted clear of the seabed or foundation the lowerpart is lifted to the surface using wires and winches (or buoyancymodules), thereby rotating the jacket about the tubular rotation beam22, before transportation to a new destination.

The lifting brackets 25 are made of steel of robust design and willabsorb all forces introduced by the lifting and rotating operations. Thelifting brackets 25 are designed to lock onto the tubular rotation beam.The lifting brackets 25 easily rotate on the tubular rotation beam 22.

Pre-engineering is required with regards to the strength of the jacketstructure before a lift can take place. The jacket legs must bereinforced if they cannot endure the loads introduced. The liftingbrackets 25 can be shaped with two long tubular clamps with a platebetween them, so that they can be mounted to the main leg and a diagonalbracing of the jacket. The brackets 25 will then absorb the forces fromthe tubular rotation beam 22 and distribute them to the tubular clamps,which in turn distribute the forces onward in axial direction of thelegs and the braces of the jacket, and so avoiding the largest shearforces. This device must be dimensioned for each individual case.

For some jackets it may be difficult to dimension the support for thebrackets 25. If this is a problem a “lifting cradle” according to theinvention can be used, see FIG. 5. The lifting cradle is attached to thetubular rotation beam 22 and uses this as a rotation point as describedabove. The cradle 29 is a framework consisting of two triangular framespointing outwards with a pointed end upwards, attached to the tubularrotation beam 22 on the pontoon. The triangular frames are connectedwith a tubular beam at the bottom of the perpendicular. The cradle 29consists of tubes 2-3 meters in diameter that are filled with water whenthe cradle 29 is in its lowest position and will be emptied when thelift starts. The large dimensions secure structural strength and enoughbuoyancy to contribute to the lift.

The lifting vessel 1 is positioned as described above and the cradle 29will embrace the jacket. Specially adjusted saddles are attached to thelower circular beam on the cradle 29, resting against the jacket legs.To avoid the jacket from sliding off the cradle 29 during the lift thejacket is connected to the tubular rotation beam 22 through bracketsattached to the jacket legs. On the back of the lifting vessel 1 winchesare mounted on each side of the “docking area” i.e. the inner area ofthe U-shaped pontoon foundation surrounded by the two longitudinalpontoons 2 a, 2 b and the transversal pontoon 2 c. Winches onboard tugscan also be used. Through pulleys wires with a hook in one end is hookedto the lower corners of the cradle 29. The cradle 29 is now liftedupwards rotating about the tubular rotation beam 22 and the jacket islifted out of the water for safe transportation to shore. An alternativemethod is to ballast/deballast the vessel 1 combined with the use ofbuoyancy modules attached to the jacket.

Lifting devices for positioning and lifting of a platform deck will nowbe described with reference to the drawings. Platform decks exist indifferent sizes and to be able to handle them all, the lifting devicemust be large, strong and flexible/adjustable, with strict requirementsto the shape for positioning around the substructure carrying the deck.

Lifting frame 12 is fitted with a horizontal robust lifting beam 13 atthe top and is pin-connected 21 to the top of the longitudinal pontoons2 a, 2 b on each side of the docking area, see FIG. 1. The lifting frame12 consists of a near-horizontal structure 18, preferably a trussstructure, going from the horizontal lifting beam 13 to the upperanchorage point 10 on the lifting vessel 1. Furthermore the liftingframe 12 consists of a vertical support structure 16, preferably atruss-work, connected in its upper end to the lifting beam 13 andconnected in its lower end to the lifting vessel through an anchoragepoint 11, preferably a pin connection 21. The lifting frames 12, 12 inthe upright position stands taller than the top of the lifting vessel 1,such that the lifting beams 13, 13 are always above the hull of thelifting vessel 1. The lifting frames 12, 12 can, with the use of thehydraulic cylinders 20, 20 connected to the lifting vessel 1 and thelifting frames 12, 12, see FIGS. 1a and 7, be inclined towards themiddle of the docking area to position the lifting beams 13, 13 underthe lifting points on the platform deck. The two lifting frames 12, 12can be run independently. The lifting frames 12, 12 are locked in theright position before the lift starts, with hydraulic bolts 9 throughholes 8 in guide rails 7 connected to each of the four columns 5 on thehull of the lifting vessel 1, see FIGS. 1 and 8. This ensures fixationin all directions included sea fastening during transport. Plane outerwalls 6 tangentially fixed to the columns 5 are supporting the guiderails 7. The plane walls 6 are furthermore perpendicular to thedirection of the connection line between two columns 5, 5.

The connection between the lifting beam 13 and the deck can be carriedout in different ways. Below is described three ways that ensuresadequate flexibility to absorb shocks during a lift off:

i) The lifting beam 13 can be equipped with a shock absorbing cover 14while also placing shock absorbing cushions underneath the deck. If itis not possible to lift directly underneath the deck the upper part ofthe jacket can be fitted with brackets 26 with shock cushions so thatthe lifting beam 13 can get a proper hold, see FIG. 9. Prior to lift offthe jacket will be cut right below the brackets 26.

ii) Hydraulic cylinders 30 are placed on top of the lifting beam 13 inwell calculated positions to get direct contact with the lifting pointson the deck structure (or brackets 26 on the upper part of the jacket).Shock absorbing cushions are placed between the deck structure and thehydraulic cylinders 30 to obtain maximum damping, see FIG. 10.

iii) “Shock cells” consisting of cylinders 35 filled with sand oranother shock absorbing material is placed on top of the lifting beams13 in well calculated positions. Conical tube stubs 37 are placed incorresponding positions on the deck structure. The conical tube stubs 37absorb shocks when they penetrate the sand-filled cylinders 35, see FIG.11a. An alternative is that both the tube stubs 37 and the shock cells35 are mounted on the deck structure, see FIG. 11b.

The MPU 1 is positioned around a jacket structure with deck and is madeready for lift off and removal of the deck. The lifting frames 12, 12 oneach side of the docking area is actively used for positioning byinclining them against the jacket with the help of hydraulicallycontrolled arms 20, see FIG. 2. Additionally the positioning is done bytugs. The lifting frames 12, 12 are pulled back into lifting positionwhen the MPU 1 is in the right position, as described above. The MPU 1is then deballasted slowly until the lifting beams 13 are touching thelifting points. Compensation for the vertical motions of the MPU 1 ispartly done by flexible shock cushions mounted on the lifting beams andlifting points, and partly by the use of a flushing system that ensuresa quick load transfer. When the deck has a safe clearance to the jacketthe MPU is pulled away from the jacket before ballasted down totransport draft.

The flushing system consists of flushing (ballast) tanks 4 above thewaterline with large area quick release trapdoors that enable the waterto flush out. Trapdoors on different levels enable multiphase flushing,i.e. flushing in several steps.

This example describes the operations for removal of a platform deck.The different operations are illustrated in a sequence of figures; FIGS.12-15:

i) Positioning around a jacket with a deck.

With the help of tugs the MPU 1 is positioned around the jacket. Thelifting frames 12, 12 are in upright position with good clearance to thejacket. The draft of the vessel 1 ensures good clearance to the deck,see FIG. 12.

ii) Using the lifting frames 12, 12 to fine adjust the position aroundthe jacket.

When the MPU 1 is approaching the correct position the lifting frames12, 12 are inclined against the jacket to dampen the horizontal motionsof the MPU 1 and also to fine-adjust the position. This is done byactive use of hydraulics, see FIG. 13.

iii) Deballasting the MPU 1, ready for lift-off.

The MPU 1 is deballasted while the lifting frames 12, 12 glide along thejacket structure to dampen the horizontal motions. The deballastingproceeds until the lifting frames 12, 12 are right under the liftingpoints on the deck. The lifting frames 12, 12 are then locked intoposition and MPU 1 is ready for lifting off the platform deck, see FIG.14.

iv) Lift-off of the deck

When the MPU 1 is ready to lift off the deck, water in the flushingtanks 4 are let out quickly by opening the quick release trapdoors inthe columns 5 thereby achieving a rapid lift. The deck is prepared inadvance by cutting the connections between the deck and the jacket, seeFIG. 15.

v) Ready for transportation to shore

After lift-off the MPU 1 is pulled away from the remaining jacket. TheMPU 1 is deballasted down to transportation draft when it is clear fromthe jacket. If necessary additional sea fastening to the locking of thelifting frames 12, 12 are added and the transportation to shore canstart. It is also possible to transfer the deck to a barge fortransportation to shore so that the MPU 1 is immediately available fornew operations (e.g. removal of the jacket).

This example describes the operations for removal of a jacket structure.The different operations are illustrated in a sequence of figures; FIGS.16-20:

vi) Positioning around a jacket (without a deck).

With help from tugs the MPU 1 is positioned around the jacket. Thelifting frames 12, 12 are in upright position with good clearance to thejacket, see FIG. 16.

vii) Using the lifting frames 12, 12 to fine adjust the position aroundthe jacket.

When the MPU 1 is approaching the correct position the lifting frames12, 12 are inclined against the jacket to dampen the horizontal motionsof the MPU 1 and also to fine-adjust the position. This is done byactive use of hydraulics, see FIG. 17.

viii) The MPU is inclined and deballasted, ready for lift-off

The MPU 1 is inclined and deballasted until the tubular rotation beam22, situated on top of the transversal pontoon 2 c, gets a hold of thebrackets 25 pre-installed on the jacket, see FIG. 18.

ix) Lift-off

When the MPU 1 is ready to lift off the jacket, water in the flushingtanks 4 are let out quickly by opening the quick release trapdoors inthe columns 5 thereby achieving a rapid lift. The jacket is prepared inadvance by cutting the jacket legs, piles, risers etc., see FIG. 19.

x) Tilting of the jacket, ready for transportation

After lift-off, the jacket is rotated to a near-horizontal position withthe use of winches and wires mounted on the aft of the MPU 1 or winchesand wires onboard tugs, see FIG. 20. An alternative method is to attachbuoyancy modules to the jacket. After sea fastening the transportationto shore can start. An alternative is to transfer the jacket to a bargefor transportation to shore so that the MPU 1 is immediately availablefor new operations.

What is claimed is:
 1. A lifting vessel (1) for positioning, lifting andhandling a marine structure by establishing a connection between thelifting vessel (1) and the marine structure, the lifting vessel (1)comprising a lower U-shaped pontoon foundation (2 a, 2 b, 2 c), andcolumns (5) attached to the pontoon foundation (2 a, 2 b, 2 c) forextending upwards towards and through a water surface, characterised byeach of the columns being free-standing above the pontoon foundation (2a, 2 b, 2 c) and a number of the columns (5) being equipped with anexternal plane wall (6), tangentially arranged on the number of thecolumns.
 2. A lifting vessel (1) according to claim 1, characterised bythe number of columns (5) being four.
 3. A lifting vessel (1) accordingto claim 1, characterised by the pontoon foundation (2 a, 2 b, 2 c)being equipped in file lower area with a horizontal brim (3) attached toat least pats of the periphery of the pontoon.
 4. A lifting vessel (1)according to claim 2, wherein the tubular rotation beam (22) is mountedon top of a transversal pontoon (2 c) of the pontoon foundation.
 5. Alifting vessel (1) according to claim 1, characterised by the plane wall(6) being arranged perpendicular to a connection line between two of thecolumns (5).
 6. A lifting vessel (1) according to claim 1, characterisedby the plane walls (6) being equipped with upper and lower anchoragepoints (10, 11) for lifting devices in respectively the top and bottomarea of the plane walls (6).
 7. A lifting vessel (1) for positioning,lifting and handling a marine structure by establishing a connectionbetween the lifting vessel (1) and the marine structure, the liftingvessel (1) comprising a lower U-shaped pontoon foundation (2 a, 2 b, 2c), and columns (5) attached to the pontoon foundation (2 a, 2 b, 2 c)for extending upwards towards and through a water surface, characterisedby each of the columns being free-standing above the pontoon foundation(2 a, 2 b, 2 c) and the columns (5) being equipped with upper and loweranchorage points (10, 11) for lifting devices in respectively the topand bottom area of the columns (5).
 8. A lifting vessel (1) according toclaim 7, characterised by the upper anchorage points (10) consisting ofguide rails (7) with holes (8) and locking bolts (9).
 9. A liftingvessel (1) according to claim 7, characterised by the lower anchoragepoints (11) being articulated.
 10. A lifting vessel (1) according toclaim 7, characterised by at least two adjustable lifting frames (12,12), each able to incline towards the middle of the docking area,consisting of a horizontal upper lifting beam (13) situated on a levelabove the top of the lifting vessel (1), a framework (16) attached tothe lifting beam (13) in the upper end and in the lower end hinged (21)to the lifting vessel (1) and an approximately horizontal structure (18)which in one end is connected to the lifting beam (13) and in the otherend adjustably connected to the columns, i.e. through the guide thesupport frames (16).
 11. A lifting vessel (1) according to claim 10,characterised by hydraulic arms (20), spanning between the liftingvessel (1) and the support fame (16), mounted in an area above thearticulated anchorage points (11).
 12. A lifting vessel (1) forpositioning, lifting and handling a marine structure by establishing aconnection between the lifting vessel (1) and the marine structure, thelifting vessel (1) comprising a lower U-shaped pontoon foundation (2 a,2 b, 2 c), and columns (5) attached to the pontoon foundation (2 a, 2 b,2 c) for extending upwards towards and through a water surface,characterised by each of the columns being free-standing above thepontoon foundation (2 a, 2 b, 2 c) and a tubular rotation beam (22)mounted on top of the pontoon foundation (2 a, 2 b, 2 c).
 13. A methodfor positioning, lifting and handling a platform deck with the use of aballastable lifting vessel (1) by establishing a connection between thelifting vessel (1) and the platform deck wherein the lifting vessel (1)comprising a lower U-shaped pontoon foundation (2 a, 2 b, 2 c), and anumber of vertical columns (5) attached to the pontoon foundation andextending upwards and through the water surface, the method comprisingthe following steps: the lifting vessel (1) is positioned around aplatform comprising a deck and jacket by the use of tugs, andfurthermore characterised by exact positioning of the lifting vessel (1)is carried out by operating hydraulically operated lifting frames (12,12) of the lifting vessel (1) towards the lifting vessel (1) middledocking area, the lifting frames (12, 12) are locked in position forlifting off the platform deck, ballast water is let out of flushingtanks by opening a quick release trapdoors, whereupon the platform deckis swiftly lifted off the jacket; and the lifting vessel (1) with theplatform deck is pulled away from the jacket and transported to itsdestination.
 14. A method for positioning, lifting and handling aplatform substructure, preferably a jacket, with the use of aballastable lifting vessel (1) by establishing a connection between thelifting vessel (1) and the platform deck wherein the lifting vessel (1)comprising a lower U-shaped pontoon foundation (2 a, 2 b, 2 c), and anumber of vertical columns (5) attached to the pontoon foundation andextending upwards and through the water surface, containing thefollowing steps: the lifting vessel (1) is positioned around the jacketwith the use of tugs, and furthermore, characterised by exactpositioning of the lifting vessel (1), until a transversal pontoon (2 c)bear against one side of the jacket, carried out by inclininghydraulically operated lifting flames (12, 12) towards the liftingvessel (1) middle docking area, the lifting vessel (1) is deballasted tocorrect draft and a tubular rotation beam (22) situated on top of thetransversal pontoon (2 c) gets a hold on lifting brackets (25) installedon the side of the jacket facing the transversal pontoon (2 c),whereupon the lifting brackets (25) is locked to the rotation beam (22),ballast water is let out of flushing tanks by opening quick releasetrapdoors, whereupon the jacket is swiftly lifted off the seabed, thejacket is rotated about the tubular rotation beam (22) with the use ofwires and winches and/or buoyancy modules, and the jacket is transportedin an approximately horizontal position to its destination.